Exam 3-solutions - Version 064 – Exam 3 – sutcliffe –...

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Unformatted text preview: Version 064 – Exam 3 – sutcliffe – (51045) 1 This print-out should have 26 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points Which of the following mixtures would pro- duce a buffer? I) 100 mL of 0.1 M NH 4 Cl + 75 mL of 0.1 M NaOH II) 100 mL of 0.1 M NH 3 + 100 mL of 0.5 M HCl III) 100 mL of 0.1 M NaCl + 50 mL of 0.2 M HCl IV) 100 mL of 0.1 M HClO 2 + 100 mL of 0.1 M NaClO 2 V) 100 mL of 0.1 M NH 3 + 75 mL of 0.1 M NaOH 1. I and IV only correct 2. I, IV, and V only 3. I only 4. I, II, III, and IV only 5. II and III only Explanation: A buffer contains a weak acid or weak base, plus the salt of that weak acid or base; or, a mixture which will have this composition after any acid-base reactions occur. You may have to calculate the number of moles of each species to determine the composition after any acid-base reaction. 002 10.0 points In the titration of a weak acid with a strong base, why is the solution NOT neutral at the equivalence point? 1. The anion of the salt that is formed can hydrolyze making an acidic solution. 2. The cation of the salt that is formed can hydrolyze making an acidic solution. 3. The cation of the salt that is formed can hydrolyze making a basic solution. 4. The anion of the salt that is formed can hydrolyze making a basic solution. correct 5. There is leftover base since it is stronger. Explanation: The titration product is the anion of the salt of the weak acid which hydrolyzes to give OH − . Titration: HA + OH − → A − + H 2 O Hydrolysis: A − + H 2 O ⇀ ↽ HA + OH − 003 10.0 points This is your BONUS question. The K b for NH 3 is 1.8 x 10 − 5 . Think about whats in there before you start the math! If a solution is 0.085 M in Mg(NO 3 ) 2 , 0.080 M in aqueous ammonia and 3.0 M in NH 4 NO 3 , what is the pH? 1. 7.70 correct 2. 6.30 3. 5.76 4. 8.21 Explanation: [Mg(NO 3 ) 2 ] = 0.085 M [NH 3 ] = 0.080 M [NH 4 NO 3 ] = 3.0 M K b = 1 . 8 × 10 − 5 Mg(NO 3 ) 2 → Mg 2+ + 2 NO − 3 NH 4 NO 3 → NH + 4 + NO − 3 NH + 4 + H 2 O ⇀ ↽ NH 3 + H 3 O + K a [NH + 4 ] = K w K b [NH 3 ] , so pH = p K a + log parenleftbigg [NH 3 ] [NH 4 ] parenrightbigg =- log parenleftbigg 1 . × 10 − 14 1 . 8 × 10 − 5 parenrightbigg + log parenleftbigg . 08 3 . parenrightbigg = 7 . 68124 004 10.0 points Version 064 – Exam 3 – sutcliffe – (51045) 2 How many endpoints would be observed in a titration of the triprotic acid (H 3 A)? 1. 1 2. 2 3. 4 4. None of these 5. 3 correct Explanation: H 3 A → H + + H 2 A − H 2 A → H + + HA 2 − HA − → H + + A 3 − These three dissociation equations show that three endpoints will be seen. 005 10.0 points For the weak diprotic acid system, which of the following statements is NOT true?...
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This note was uploaded on 11/17/2011 for the course CH 302 taught by Professor Holcombe during the Spring '07 term at University of Texas at Austin.

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Exam 3-solutions - Version 064 – Exam 3 – sutcliffe –...

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